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Jan.27,1931. NTRBOJEV'CH 1,790,606-

WORM GEARING Filed May 14, 1928 3 Sheets-Sheet l @d l l y y Jan- 27, l9341# N. TRBoJEvlcH 1,790,606

WORM GEAR I NG Filed May 14 1928 s sheets-sheet' 2 Y f 5 47 ff? I ,QW/,

` Jan. 27, 1931. l N TRBOJEWCH 1,790,606

May 14: 192,8

Patented Jan. 27, 1931 NiKoLA TRBoJEvIci-i, or DETROIT, viviiciir'eaii WORM creatureA i Application led May 14; 1928. Serial No. 277,693.

The invention relates to a novel type ot worm gearing which is particularly adapted to the axle drives of automotive vehicles and' othei' such mechanisms.

struct the worin and cooperatinggear that an increased area of contact is obtained or` in other words, to increase the number o'f teeth which are .simultaneously in engagement. A

The object'of the invention is to so coif-y further objectis to provide a construction of v the above type wherein the teeth are of such forni as to be capable of being manufactured by commercial methods, that is` by accurately milling, hobbing, shaping, etc. To attain the above mentioned ob]ects. I

-have provided a new type of worm gearing wherein the driving member or worin is ot l 'an liour-glass or of a globoid shape and has a plurality of threads which are tapering from the gorge or central plane in both directions at their peripheral boundary. The cross coutour of the tapering threads also changes from point-to-point according to a predetermined law, the arrangement being such that the tooth of the wheel which mates with the worm thread shows a bearing or contact surface A composed entirely ot a family of circular helixes all having the same lead and all concentric about tlie same axis.

In the drawings- -F'gures l and 2 are geometrical diagrams showing the manner in which an increased area of contact is obtained;

Figure 3 is the elevation of the improved worm and gear;

Figure 4 is the plane section 4-4 of the Figure 3,; y

Figure 5 is a diagram showing the location of the parallel planes in which the tooth contact may be analyzed;

Figure 6 is,the plane section 6 6 of the Figure 5;'

Figure 7 is the plane section 7-7 of'the' Figure 5;

Figure'S lis the plane section 8-#8 of the Figure -5;

`Figure 9.is the planview of a liobbing machine capable of generating the new hourfglas'sworm;

Figure 10 is a detail view of Figure!) taken in the plane 10-10.

Figures 11 and 12 show the method ot' sharpening and relieving respectively the cutting teeth of the cutter shown in Figures 9, 10. 13 and 14;

Figure 13 is a diagrammatic view of a modified Fellows gear shaper to cut hour-glass worin; l

Figure 14 shows a inodiication of the machine represented in Figure 13 in which the cutter is mounted eccentrically;

Figures 15 and 16 'are diagrams showing two principles of constructing the tooth curves of worm andgcar; Y

Figure 17 diagrammatically shows the method of grinding the new worms.

Figures l'and 2 diagrammatieally show the benefit which is obtainable by the use of the new hour-glass worm gearing in comparison Wit-h the common or straight worm type. In the present worm driven automobile axles the driving member 21 is usually considerably smaller in diameter than the driven member 22 in order to obtain the required reduction or ratio, said ratio ranging from about four to fifteen. 'Heretofoie the worm number 21 has usually been maderst'raight and the gear of a hollow or globoid contour. Iiiiny in'iproved construction' the worm is made of globoid form and the gear straight. Assuming an angle a inside of 'which a contact is obtainable for a given pressure angle of gear teeth `it will ,be seen that thecontact .area in the straight type worm is represented in Figure 1 by the circular segment A, B, and C, and in Figure 2 by the sector D, E, F and G, thiis giving the volume of Contact in the form of a saddle shaped solid. By employing an hour-glass worm 23 and by leaving the other dimensions including .the limiting au gle a intact it is seen that the area of conta :t in Figure 1 is considerably increased to form the sector H, I, J and K, while as shown in Figure 2 it is only slightly decreased as the two upper corners of the area D, E, F and G lying above the outside diameter 24 of the gear 22 have now been lost. The gain in the volume of contact is thus easily demonstrable whenever the worm is smaller than the wheel.

Figures 3 and 4 show the new worm drive `Ain two projections. rlhe .driven member 25 is a cylindrical gear having a plurality-oi helical teeth 26, said teeth forming in any plane section parallel to the axis 27 a seriesv of racks of constant pitchi28. Said racks are fot, a symmetrical tooth form in any axial plane (see Figures 4 and 6). and are lopsided inaiy olf-set plane, Figures 7 and 8.

rlhe tooth curves ot said racks may be straight lines or curves according to as de? sired' and as hereinafter pointedoi-it there i is a particular advantage obtained by making said curves concave. f Y

4The driving member' 29 Iis an hour-glass worm having its spiralv teeth 30 so formed that'the tops ot the Vsaid teeth are tliewidest in the midplane 4-4 and gradually taper oft toward the right and left extremities of the Worm as markedly indicated at FV, Figure 3. Thus, the cross section of the thread is diminishing orft'apering in bot-h said directions, and the tapering is e'ected on the Hank 3l of the thread facing the midplane 4--4 while the i, stantially constant pitch.

tlank32 facing outwardly remainsof a sub- -The action of the new gearing will best be understood from Figures to 8 inclusive. Figure 5 shows diagrammatically the vgear 25 and the mating globoid worm 29 in elevation and indicates the position of-the parallel planes 6-6, 7-7 and 8-8 in which the sections shown in "Figures 6, 7 and 8T were respectively taken. Asthe members 25 and 29 rotate a'constant ratio and at a tixed center distance, the plane 33 represents the pitch' surface of the drive and said plane is parallel i the tooth curves 4l are generated according to the well known rack and spur gear princ-iple. y ln Figures 7 and 8 the rack elements 37 and 38, being non-.radial .sections of a helical gear, show a greater pressure angle on one. side of the teeth than'on the other, that is. they are lopsided. The pitch circles 3.9 and 40 of the laminas 42 and 43`touch the pitch` line 33 atthe points N and Q respectively .and roll over it without sliding as was also the case in the Figure 6. The essence of this "is, that due to the tact that we now have means of graphically and mathematically analyzing of just what happens in each parallelsection ot the worm 29 for a given shape -of the helical gear 25 and a given selection -to' three teeth in contact.

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of the pitcliplane 33, we may so select said tooth shapeand pitch plane that the worm 29, Figure 5, will have a tull contact tromend' to end and will have well ormedteeth showing no mutilation of tooth form due to secondary contacts at any place. Said secondary contacts manifest themselves in two ways, viz, las an vun'dercutting7 at the root diameters of the laminas 36, 42 and 43', Figures 6 to 8 or as a feather edge at the tops of the teeth of the said laminas, see'Figure 8. rlihe first defect is caused either by a too small `pressure angle at the rack element or by a vtoo great distance of the pitch plane 33v from the bottom of .the thread, Figurev 5, or both, and the second' detect is vlcaused by just the opposite ieasoiis, i. e. by too emuch `pressure angle o r an insuiiicient distance ot the. pitch plane` from ,the 4axis 34.

Thus, in designing a pair of mating gears of thisl type to obtainthe maximum area of the tooth bearing sur-face and the maximum load-carrying ability, lt proceed as follows: The center distance 'and the respective numbers of'teeth in thc worm and gear 'are usually given and the helix angle ,8, Figures 3 and 4 should be selected as nearto 45O as the strength of the worm will permit because a .greater helix angle (up to 45) gives a better efcien'cy and an easier coasting gear. From these data the distanceof the pitch plane 33 Jfrom the'axis 34, F igure 5, is fixed.v The et' feet of increasing the helix angle is to push the vplane 33 toward the axis'34 and thus-to reduce the diameter of the worm 29, the same as in common worm gearing. lAfter having the pitch plane 33 thus fixed, the next step is to build the worm thread about said pitch plane in such a manner that itwill not become undercut in the mid-plaiie, Figure 6. Having thus located the bottomof the threadfrom the point M, Figure 6, as far as possible and still avoiding undercut, we investigate thc feather edge conditions in the diagrams 7 and 8. it the result not satistactory,we change the pressure angle and the helix angle ,8 until a greater eiective len'gthof worm is obtained. lit the featheredge starts too soon (in other Words if there is insutcie'nt contact area between worm and gear) it is necessary to lessen the pressure angle or the helixangle or both until greater contact areafis produced. ln such a manner, foixinstance, l siicceeded in constructing a drive, 8 into 32'teeth, 30olielix angle, 160 pressure angle, ywhich showed a full bearing extending over,v six teeth whereas a conventional worm gear of the saine proportions would show only two The methods of manufacturing vthe vnew gearing will now be described. ,The method of making the gear 25, Figures 3 and v4, presents no diiiculty as that member beinga helical gear of a constant pitch and lead may be hobbed, shaped, milled or ground accordaoy ing to any one of the well known conven-A tional methods. Y

The worm 29 is manufactured by a process which I discovered and which -is some1 what similar to bobbing. Figure 9 represents a plan View of a common gear bobbing machine in which this process may be performed.' The worm 29 is mounted upon the hob arbor 44 where ordinarily-the hob is, and the cutter 45 is placed upon the Work arbor 46, i. e., just the opposite of the conventional practice. When the arbor 44 is rotated, the motion of the Worm 29 is transmitted to the cutter 45 in the timed relation through the bevel gears 47 and 48, the index change gears 49, the worm 50 and the worm gear 51. At the same time, the feed mechanism is actuated in a timed relation through the feed gears 52 and the feed screvsr 53, said feed screw engaging a corresponding nut in the slide 54 thus causing the same to move slowly over the ways 55 toward the cutter 45. The shaft 56 carrying the bevel gear 47 is splined in order to permit an uninterrupted rotation J "of the parts at all times and positions.

The axis 27 of the cutter 45 is situated above the axis 34 of the worm at the exact center distance Q required for the finishing out` as shown in detail in Figure 10. It is of interest to note that in this construction the worm may be lifted out of the cutter or gear Without causing the so-called assembly interference or mutilation of the tooth surfaces as is the case with the common Hindley worm.

' The cutter 45 is similar in dimensions to the helical gear which it is desired to imitate in action such .as the gear 25 in Figures 3 and 4. The bobbing machine shown in Figure 9 is so geared up b v means of the change gears 49' and 52 that the cutter v45 will move relative to the blank 29 in an exactly predetermined helieoidal path. The teeth 57 of the Vcutter 45, Figure 11, are sharpened at their ends at the plane faces 58 preferably at right angles to the helix and they are relieved about the circumference and also along the side flanks 59 as indicated in Figure 12.

It is thus seen that the cutter employed in this process is of the same construction as the Well known Fellows cutter. If the tooth curves and the diameter of the cutter are exactly the same as of the gear which it replaces, if the lead of feed exactly corresponds to the lead of said gear and if, by a proper design of tooth curves andthe selection of the pitch planeas Was pointed out in a previ- A ous paragraph, the mutilation of the generated surfaces in the Worm is prevented, a virtually perfect hour-glass worm Will be ob- .t-ained having a full bearing from end to end and a line contact with the mating gear in each and every engaging tooth and at every instant.

Another method of generating the new hour-glass Worm is shown in Figure 13. ln View of the preceding, the arrangement need be only briefly described. The cutter 45 is mounted at the end of the ram 60, the upper part 6l of said ram being helically fiuted to the exact lead of the cutter and the lower part 62 being of the shape of a circular rack. The gear segment 63 oscillates'to and from and thereby causes the cutter 45 to reciprocate up and down. The helical part- 61 of' the ram engages the corresponding nut 64 which is rotatably held in the rigid frame 65 of the machine. The worm gear 66 is integral with the said nut (i4 and is rotatable by means vof worin 67 and the gears 67. 68, 69, 7() and 7l. The worin 29 is also connected to the same train of gears by -means of the shaft 72 so that a timed relation exists between the inembers 29 and 66. The action of this machine is easilyv understood. Assuming the rain 60 to reciprocate up anddown the train of gears 66 to 7l n-iay.' be imagined as standing still and the cutter'i 45 will unscrew itself in and out in the nut 64 and its cutting edges will describe the helical tooth surfaces of the gear which it represents. If now the gear train 6 to 7l is actuated, the cutter.45 will rotate about its axis at the same rate of speed as the worm Vgea-r 66 and will therefore -meshwith the previously that the cut-ter 45 is an exact duplicate of the gear 25, Figure 3. It may happen, however, that said gear is rather large, and acorrespoiulingly large cutter would be difficult and expensive to construct. lVith the object of overcoming this disadvantage I have devised a process in which a small cutter may be used for the'manufa'cture of large Worms.

The helical bore 74 in which the ram of the` cutter73 now reciprocates is eccentric relative to the circle of the gear 66.- The members 67 and 29 are rotated in a timed relation as before. By this method only one particular tooth 75 of the cutter 73 will' cut to the full depth of' the worm thread and in order to finish all of the threads of the worm 29,the cutter 7 3 should be indexed once for each thread of the said Worm.

In another modification I make the helical bore`74 in the form ofa rotatable cylinder embeddedin the body of the gear 66.

fand rotate said cylinder in a timed relation' with the worin 29. I n that case the process of' generation will be continuous and no indeXing for each thread of the worin will be required as it will be understood.

result that the corresponding curves Y? of the worin 29 will become concave. l use this method of forming the tooth curves when there is no need of `grinding the worin threads after hardening.

flu Figure 1,6, the tooth curves TS of the gear are concave and the mating curves T) ot the worin are-coiivex.' ln. a special case7 said curves T8 may be so developed that the mating curves T9 will bcconie straight lines in section perpendicular to the'helix' of the worin. ln such a case. sec Figure 17.

a grinder 80 having a straight V shaped` protile 81 and set perpendicular toy thel uor.` vmal plane'SZ may be usedto grind two adjacent worin threads U3 on their adjacent .'sides ln oider to grind the worin threads and in addition it must be oscillatedin a helical path above and below said plane 84. 'lhis may be done by a mechanism similar to one shown in the Figure ll as it will bcunder-v stood. v

Referring now again to Figure El it is seen that the' chief characteristic ot the new worin 29v is in that its threadsare'ot a tapering and variable crosssection which tact neces sarilyfollows from vthe i'nethodof generation. As the generating tool .ot a constant crosssection reciprocates'in al helical path aboveand below the4 central plane ot thek wornnthel pitch line 33, Figure is vsuppressed toward the roots oi' the thread at bothlarge ends of the Worm, thusproduciug a tapering thread. Besides.providing a full contact et theworin thread with the mating gear teeth this' construction also permits oi' v mounting the mating'` gears at slightly variable center distances. lt', tor instiince. the center distance is slightly increased some' of the bearing ai'ea Willbe l st,\liowever, the gears willanesh at a constant velocity and withoutinterference. x

, llVhen especially free and tast running gears are required,` ll propose to 2generate the worm with a 'cutter larger in diameter than the corresponding mating gear. lnsuch a` case the bearing is'slightly f-eased ed toward `Jthe large ends ot the Worin thus providing a gradual approach for the entering teeth. This l accomplish by adding-one or more teeth iii the cutter above the .corresponding number of teeth iii the matingbvvheelr if the Worin is generated by method shown in Figure 9, or by suitably increasing the radius of' the circle 66, Figure 14, it the Worm-be genacute angle. 1t the shaft' angle be suitably.- selected the driven member may be aspur having straight teeth.

erated by the method ot the'eccentrically'-l I l llt will be clear from the above description that compared with those of conventional forms the new Worin gear odors a number ot salient advantagcs, all the more 'important as they can be secured' Wit-hout appreciableA y increase in the .costoi production. The new worin when constructed With an iiivolute tooth torinybehaves in manyrespects simi-v larly t-o-an internal helical gear .in thatv it will correctly mesh with all other involute i helical gears of `the saine pitch and pressure angle having a pitch radius less 'than that of the circle 66, Figure 111,v and Will also correctly mesh With' anyone of such gears inan .offset position (offset being measured from -thegorge plane-ot the Worm) as clearly into an exact contour, the grinder 8O must be.- translated in a circular path iii-the plane 84 principle inthat they will correctlyf`mesh A even i'n the case when neither the Worm nor the wheel are exactly centered along their respective axes and the center distance itselt1 may be varied Within a certain latitnde Without violating the said involute \principle',

similar to other kinds or invohite'gearing These advantages are'of'great practical value when it comes to mass production 'as my de sign provides liberal manufacturingl'tolerancesbotli in manufacturing and assembling and thus renders the globoid principle available, for the first time to my knowledge, :to a cheap. rapid and accurate'production ot worin gearing.

Another ,advantage of the involute.` glo.

boids.' seev Figure 15,' is due to the :tact that the flanks 1T ot the Worm are concave-and of a variable radius ot curvature. This feature makes the Worm thread Wider'at the roots (compare with Fig. 16)Aa1' id also makes the lines of pressure (which are perpendicuf lar to the cur-ves '(7) converge .at a deiiiite distancerom the Wheel axis. `For these two reasons the new Worin is Well, adapted to ab-v sorb rapidly*alternating loads andshocks.

l 1What l claim as my invention is 1. A Worm comprising a body .ot hourglass ,shape having a spiral thread ,woundy thereon` said thread being'sojtormed that the side facing lthe gorge plane has a varying' lead increasing from the gorge plane toward the outer ends-said. thread having the opposite'sides formed with av substantially constautlead.'

2. ln an hour-glass Wormfthe meridian curve of Which is a sectionof a cylinder,a

doubly tapering spiral thread so formed that the thread diminishes in Awidth from each side of the gorge plane and its contour .changes from point to point in such a Inany gear having their axes non-intersecting and y non-parallel in which the tooth surfaces of the gear are representable by a sweeping motion alone of 'a master curve in a path along the axis of the gear and the tooth surfaces of the worm are representab'le by the combi-nation of the said sweeping motion of the said master curve in the' said path with an intermeshing rolling motion obtained by rotating the worm and the said master curve about their respective .axes in a timed relation.

4. A worm drive consisting of a smaller globoid wormvand a larger helical'gear having their axes disposed in two planes interse'cting at a right angle in which the tooth surfaces ofthe gear are ,representable by a sweeping motion alone of a master curve in a helical path about the axis of the gear and the tooth surfaces o f the worm are representable by the combination of thesaid sweepf ing motion of the said master curve in the saidhelical path with an intermeshing rolling motion obtained by .rotating the worm and the said master curve about their respective axes 1n a timed relation.

5. A globoid worm having spiral teeth of.y

a contour generated by rotating the worm about its axis, by rotating Aa predetermined master curve in atimedsrelation about another axis, said 'second' axis being non-intersecting and nonparallel relative to the first axis and by translating the said master curve parallel to itself in a direction transverse'to the axis of the worml 6. A globoid worm havingspiral teeth of a contour generatedby rotating the worm about its axis, by rotating a predetermined master curve in a timed relation about another axis at right angles to the first axis and'by translating the said master curve parallel to itself in a direction transverse to the first axis.

7. A globoid worm having spiral` teeth of an involute contour, said contour being` obtained by rotating an involute of a circle in a timed relation about its axis, said axis being non-intersecting and non-parallel relative to the first axis and by translating vthe said' involute parallel to itself in the direction of the second axis. A I

8. A. globoid worm having spiral teeth of an involute contour, said contour being oba-timed relation about its axis, said axis being non-intersecting and non-parallel rela- -tive tothe first axis'andbytranslating the saidinvolute parallel to itself in the direction of the second axis and in a helical path.

9. An'involute globoid worm having a predetermined pitch and pressure angle and-a tooth contour capable of correctly meshing with an involute gear lamina of the same Y pitch and pressure Vangle when the said ,laminaV is placed in the axial plane of the worml tangentially relative to the worm threads and is rotated simultaneously about its own axis vand also about the axis from whichfthe meridian. circle of the worm is drawn. l y i l 10. An involute globoid worm having spiral teeth of ari-involute contour capable of correctly meshing with an involute spur Vgear lamina. of the same pitch and pressure angle as the worm when the. said lamina is placed tangentially in the axial plane of the worm and is oscillated up. and down along its axis in a helical path. 1l. A. gear `drive comprising a smaller member of globoid and larger'one of a cylindrical-contour, the former having generated teeth of variable cross section but'all derived from a predetermined base cylinder and the latter having equispaced involute teeth of a constant cross section aligned along a plurality of helixes drawn from a common base cylinder, the geometrical' relationbeing s uch that the pitch and pressure angle are the same in both members, and the base cylinder Qofthe globoid membe'ris always vequal to Vor greater than the base cylinder ofthe wheel member" to provide` a predetermined degree of enveloping'contact.

12.'A globoid worm comprising a plurality of spiral thread convolutions 'of a concave thread cross contourin the axial plane thereof in which the radius of curva-l ture of the said cross contour gradually increases' asmeasured from the tips toward the Aroots of the said thread and capable rof cordeveloped from a basccircle which is con centric with the meridian pitch circle of the worm and of a less radius than the said meridian circle.

In testimony whereof I aihx my signature.

NIKoLA rRBoJEvicH.

tained by rotating an involute of a circle in 

